The central nervous system is a vital part of the human physiology that coordinates human activity. It is primarily made up of the brain and the spinal chord. The spinal chord is made up of a bundle of nerve tissue which originates in the brain and branches out to various parts of the body, acting as a conduit to communicate neuronal signals from the brain to the rest of the body, including motor control and sensations. Protecting the spinal chord is the spinal, or vertebral, column. Anatomically, the spinal column is made up of several regions, including the cervical, thoracic, lumbar and sacral regions. The cervical spine is made up of seven vertebrae and functions to support the weight of the head. The thoracic spine is made up of 12 vertebrae and functions to protect the organs located within the chest. Five vertebrae make up the lumbar spine. The lumbar spine contains the largest vertebra and functions as the main weight bearing portion of the spine. Located at the base of the spine is the five fused vertebrae known as the sacrum. The coccyx sits at the base of the spinal column and consists of four fused vertebrae.
Each of the vertebrae associated with the various spinal chord regions are made up of a vertebral body, a posterior arch, and transverse processes. The vertebral body, often described as having a drum-like shape, is designed to bear weight and withstand compression or loading. In between the vertebral bodies is the intervertebral disc. The intervertebral disc is filled with a soft, gelatinous-like substance which helps cushion the spine against various movements and can be the source of various diseases. The posterior arch of the vertebrae is made up of the lamina, pedicles and facet joints. Transverse processes extend outwardly from the vertebrae and provide the means for muscle and ligament attachment, which aid in movement and stabilization of the vertebra.
While most people have fully functional spinal chords, it is not uncommon for individuals to suffer some type of spinal ailment, including spondylolisthesis, scoliosis, or spinal fractures, which are so severe as to require surgical intervention. One of the more common disorders associated with the spinal chord which requires surgical intervention is damage to the spinal discs. Damage to the discs results from physical injury, disease, genetic disposition, or as part of the natural aging process. Disc damage often results in intervertebral spacing not being maintained, causing pinching of exiting nerve roots between the discs, resulting in pain. A disc herniation may compress the spinal cord and/or nerve roots and cause pain, loss of function, and even complete paralysis of the legs with loss of bowel and bladder control.
For most patients, rest and administration of pain and anti-inflammatory medications alleviates the problem. In severe cases, cases which have developed into spinal instability or severe disc degeneration, the damaged disc material between the vertebral bodies is removed and replaced with spinal stabilization implants. Restoration to the normal height allows the pressure on the nerve roots to be relieved. However, removal of the offending discal tissue has proven both difficult and quite dangerous. When the discs of the thoracic spine are approached posteriorly (from behind), the spinal cord is in the way. To approach the same herniation anteriorly (from the front) requires the very formidable procedure of thoracotomy (cutting open the chest) and moving the heart and lungs out of the way.
There are many different approaches taken to alleviate or minimize severe spinal disorders. One surgical procedure commonly used is a spinal fusion technique. Several surgical approaches have been developed over the years, and include the Posterior Lumbar Fusion (PLIF) procedure which utilizes a posterior approach to access the patient's vertebrae or disc space, the Transforaminal Lumbar Fusion (TLIF) procedure which utilizes a posterior and lateral approach to access the patient's vertebrae or disc space, and the Anterior Lumbar Fusion (ALIF) which utilizes an anterior approach to access the patient's vertebrae or disc space. Using any of these surgical procedures, the patient undergoes spinal fusion surgery in which two or more vertebrae are linked or fused together through the use of a bone spacing device and/or use of bone grafts. The resulting surgery eliminates any movement between the spinal sections which have been fused together.
In addition to the spinal implants or use of bone grafts, spinal fusion surgery often utilizes spinal instrumentation or surgical hardware, such as pedicle screws, plates, or spinal rods. Once the spinal implants and/or bone grafts have been inserted, a surgeon places the pedicle screws into a portion of the spinal vertebrae and attaches either rods or plates to the screws as a means for stabilization while the bones fuse.
While bone fusion technique often alleviates the pain associated with such disease, implantation of the spinal implant devices can be difficult. Gripping features, as well as various shapes, have been incorporated into the design of implants for ease of insertion and better hold on the vertebral endplates. Implantation is more difficult when the surgeon utilizes multiple implant devices, as proper alignment of the multiple implant devices can be very challenging. Implanting several implant devices can be time consuming, increasing the risk of surgical complications. Once implanted, the implant devices do not always remain at the site of implantation. Implant expulsion results from implants migrating and expelling out of the disc space, usually through the insertion pathway. Implants that migrate to the spinal chord or nerve roots can cause serious complications to the patient. While threaded features or teeth are used to prevent such migration, care must be used when incorporating these features into the design of the implant, as such features can result in localized stress risers in the end plates causing implant subsidence.
There exists, therefore, a need for an improved spinal implant that is easy to insert, is self-aligning when multiple spinal implants are used in a surgical procedure, and reduces the risk of spinal implant expulsion.